How does tapping a recently shaken up can of cola/beer
prevent it from frothing when opened?

Dear Cecil:
I have always laughed at people who, before opening a carbonated drink
which has been shaken, tap the top of the can with their finger so that it
doesn't explode upon opening.
After lengthy arguments, we even performed a semiscientific experiment by
shaking a drink and opening it with and without tapping the top, but with
no solid scientific conclusion.
We would like to know what you, in your infinite wisdom, think of this.
--Benjie Balser, Dallas, Texas
Dear Benjie:
This is not a problem that requires infinite wisdom. This is a problem
that requires an IQ above two, apparently a stretch for some folks these
days.
First I called the folks at Coke central in Atlanta. I did this in the
interest of thoroughness, in case Coke physicists had discovered quantum
mechanical aspects of beveragecarbonation that had previously eluded the
notice of science. However, they didn't return my calls.
There are two possible explanations for this: (1) everybody was out in the
plant stamping out souvenirOlympic bottles, or (2) Cecil's message was a
little too detailed.
This is an inherent risk in my business. If you tell some low-levelgatekeeper type you have a question about poultry, you may actually get
through. Tell them you want to know which end of the egg comes out of the
chicken first, on the other hand, and they'll have security trace the
call.
No matte. First let's consider the matter from a theoreticalperspective.
Carbonation is produced by forcingcarbondioxide into solution with H2O
under pressure. Shake up the can and you create thousands of micro-size
bubbles. Each bubble offers a tiny surface where CO2 can rapidly come out
of solution, creating the potential for explosivefizzing should you open
the can prematurely.
Wait a while though, and the bubbles will float to the top of the can and
disappear, and eventually all will be as before.
But suppose you're the impatient type. You tap the can.
What, pray tell, is this supposed to accomplish? Are we going to noodge
the tinybubbles to the surface faster, after the manner of herdingcows?
Right.
Are we going to maybe dislodge a few bubbles that have stuck to the sides
of the can? Maybe we are, but the difference is slight. Open that baby and
you're still going to get a faceful of froth.
We confirmed this to our satisfaction out in the Straight Dope Backyard of
Science with a half dozen cans of pop. OK, so I didn't replicate my
results 50,000 times. I figure if extraordinary claims demand
extraordinary evidence, stupid claims demand ... well, something a little
less rigorous.
I should tell you that when I had Little Ed broach this issue recently on
the Usenet he heard from a science teacher, among others, who insisted
tapping the can really did reduce fizzing and bragged about a classroom
demonstration he did to make just this point.
We wrote back to the science teacher: had he, like, tried comparing the
amount of froth from tapped vs. untapped cans--one of your more basic
experimental procedures?
Rarely, he replied. 'Nuff said.
The Usenetdebate dragged on for weeks. On the one hand there were various
learned personages offered advanced theoretical reasons why tapping should
reduce the froth. On the other hand were a hardy band of experimenters who
kept trying to find a "tapping effect" and failing.
The lack of experimental confirmation failed to deter the believers.
Finally one Martin Heinz wrote in exasperation:
"I did try some `pseudo-experiments' (thank you very much) which led me to
believe that tapping does *not* have any effect. However, the variations
are endless. People argued:
"1) It depends on *how* you tap. (Top? Side? Flicking a finger? Knocking?)
"2) Just *when* is a soda can agitated enough for the hypothesized effect
to work? (Shaking? Dropping? How much? How high? Does it count if the can
is scooped up by a plane dropped in a forest fire?)
"3) It depends on what *kind* of soda you use. I used Diet Coke, then
someone said, `Noooo, the NutraSweet throws everything off.'
"4) It depends on the temperature of the soda.
"5) It depends on the country the experiment is conducted in, because
`tapping' is one of the many words in Inuit related to soda and means
`sticking a can up your butt.'
"You be the judge."
You'd think that would have ended the debate. But no debate on the Usenet
ever ends. It just sort of peters out.
So I decided to appeal to authority. I know, I AM the authority. But I
figured it was probably smart to ask for a second opinion.
I called up physicist Jearl Walker, who's written about the physics of
beverage carbonation in Scientific American.
Jearl, you'll remember, is the guy who used to plunge his hand into a vat
of molten lead as a classroom demonstration of the Leidenfrost effect.
This makes him either certifiably crazy or a genius--in either case
somebody you want to listen to with an attitude of respect.
Jearl had heard similar claims about the efficacy of tapping and had a
similar reaction: these guys are nuts. He said he could only attribute the
persistence of the practice to the same suppressed macho ethic that makes
people tap the ends of their cigarettes before lighting up.
If you want a real solution, try this.
It's an implacable fact that a warm can of pop that's all shook up will
fizz more than a shaken cold can. If you absolutely must pop the top on
that jug of Jolt, stick it in the fridge first. You'll chill the contents
and chill the carbonation too, an inevitable consequence of increased gas
solubility and Charles's law.
Too much trouble? Fine. Just let the can sit awhile to give the
microbubbles time to disappear.
To pass the time you can even tap the top of the can. Just don't kid
yourself the tapping itself makes the bubbles go away.
--CECIL ADAMS

Beer, colas and other carbonated drinks contain carbon dioxide]
which is held dissolved in the liquid under pressure. In fact it
reacts with water to form a mildacid which improves the taste
of the drink. Provided it has been standing at the same temperature
for a long time the gas in solution will be in equilibrium with the
gas in the head-space of the can. Shaking the can will not cause
any change in pressure which can be detected by hand.

It is possible to produceplausible sounding theories which
suggest that the pressure might increase. Perhaps momentary areas
of low pressure in the can cause bubbles to come out of solution
which are not so easily redissolved under high pressure. It is
also conceivable that agitation could reverse the weak reaction
between the carbon dioxide and water causing gas to come out
of solution. To show that these theories are incorrect it suffices
to perform a simple experiment, but care is needed to avoid
a misleading result.

Mark three of the cans underneath and shake them vigorously.
The other three cans should be left undisturbed so that they can
serve as controls. Now get someone to shuffle the cans around
while you are not looking so that you no longer know which ones
were shaken. Without looking at the marks underneath, try to
pick out the three cans which seem to be at the highest pressure.
Finally look underneath to see if you managed to pick out the
three which had been shaken. Of course, you had a one in eight
chance of getting it right by luck so it may be necessary to
do it several times to improve the statistics.

The reason for doing it blind is to avoid the
possibility of misjudging the pressure in accordance with your
own preconceptions. feeling the pressure of the cans with
your fingers is very subjective and can not be relied on if
you know which cans have been shaken. This kind of experimental
mistake is partly to blame for the existence of the myth in
the first place.

So why does the can froth so violently if it is opened after
it has been shaken? This is caused by the bubbles which are
generated by the agitation. These multiply and expand when
the can is opened.

But what of that anecdote about the time someone shook
a plastic bottle of cola so hard that it burst? Well the pressure
inside a bottle of cola can be reasonably high, especially if it
is not chilled. Shaking it very hard can strain the bottle
near to its limits just because of the force of the liquid as it
hits the sides. Add to that the increased pressures caused by the
hard squeezing grip which is necessary to hold it as it is shaken,
and it is not difficult to account for it bursting.